Various methods are known in the prior art for stimulating the production of oil and natural gas from wells drilled in formations with low permeability. One of the most widely used methods involves the hydraulic fracturing of low permeability formations using one of a variety of water-containing fracturing fluids intermixed with proppants such as sand, glass beads, and the like. Hydraulic pressure is applied to the formation and causes stresses in the rock of the formation surrounding the well bore. These stresses trigger the splitting or “fracturing” of the rock. The initially formed fractures are then extended by the injection of fluids containing a proppant into the fractures. The hydraulic pressure is then released and the proppant which had been previously deposited in the fractures serves to hold the fractures open such that channels are created for reservoir fluids to flow to the well bore and subsequently to the surface.
Another prior art technique for stimulating reservoirs involves the hydraulic fracturing of low permeability formations using foam. The foam is formed by blending sand or another suitable proppant into a gelled water solution and treating the resulting slurry with a surfactant. A gas, such as nitrogen or carbon dioxide, is then injected into the slurry to create a high-pressure foam. The foam/proppant mixture is then used to fracture the formation as described above. Foam fracturing has several advantages over fracturing techniques using conventional fracturing fluids. The foam has a low fluid loss and has the ability to create larger area fractures with equivalent volumes of treatment fluid. Since fluid loss to the formation is minimized, the chance of damaging sensitive formations is lessened. Foam fracturing systems are of particular benefit in gas or oil reservoirs of low reservoir pressure.
A more recent development in well fracturing involves the use of gaseous nitrogen alone as a fracturing agent. The water content of fracturing fluids is extremely crucial when treating water-sensitive formations (such as coal bed methane formations) since water can cause clay migration and swelling in the formation, thereby reducing permeability to the produced fluids. Gaseous nitrogen, on the other hand, is inert, relatively insoluble, and compressible. Using gaseous nitrogen as a fracturing agent virtually eliminates the clean-up problems associated with water-based systems. Once the treatment is completed, the well is opened up and the gaseous nitrogen is flowed back to the surface. Production fluids are not lost and costly swabbing units used to recover the water are not needed.
Although gaseous nitrogen fracturing has many advantages over traditional water-based and foam fracturing techniques, there exists a need for a gaseous nitrogen fracturing technique that includes the simultaneous placement of an effective amount of proppant within the fractured formation surrounding the well bore. An effective amount of proppant serves to hold the fractures open such that channels are created for reservoir fluids to flow to the well bore and subsequently to the surface, thereby stimulating the well. The following invention addresses that need.